1. Field of the Invention
The present invention relates to a thermal transfer material and a laser thermal transfer recording method in which thermal transfer of an image is performed by irradiation of a laser. More particularly, the present invention relates to a thermal transfer material in which a color proof (DDCP: direct digital color proof) or a masking image in printing is formed due to irradiation of a laser on the basis of digital image signals, and to a laser thermal transfer recording method.
2. Description of the Related Art
There is a thermal transfer and recording technique, in which a thermal transfer image receiving material, and a thermal transfer material having a support on which a color material layer is provided, are laminated to each other. The color material layer contains therein a thermally soluble color material layer or a thermally sublimating dye. The laminated thermal transfer image receiving material and thermal transfer material are heated imagewise from the thermal transfer material side by using a heating device which is controlled by electric signals, such as a thermal head, or an electrically conductive head to thereby transfer and record an image onto the thermal transfer image receiving material.
Such a thermal transferring and recording technique has characteristics of low noise, being maintenance free, having low manufacturing cost, facilitating coloring, and being capable of digital recording. This technique is therefore utilized in multiple fields such as in various types of printers, recorders, facsimiles, and computer terminals.
On the other hand, in recent years, in the medical and printing fields, demands have been made for recording systems which have a higher resolution and enable high speed recording as well as enabling image processing, i.e., recording systems which enable so called digital recording. However, in the thermal transfer and recording system in which a heating device such as a thermal head or an electrically conductive head is used, image resolution of this system is constrained by the layout density of the heating elements of a head. Further, it is difficult to control the heating temperature of the heating elements at a high speed, due to the characteristics of the heating elements. Accordingly, it is difficult to obtain a high resolution image at higher speed.
One system capable of providing an image with higher resolution at higher speed is a laser recording technology which utilizes a light-to-heat conversion action due to the irradiation of a laser. Recently this system has attracted much attention and is being manufactured as a finished product.
In an image forming system using this technology, in particular, the single mode laser is generally used from the standpoint of attaining highly accurate and finely focused beams, and due to such beam quality, a high resolution image is obtained. On the other hand, although recording speed is also improved such that an image is formed more speedily than in a conventional recording system which uses a heating device such as a thermal head, since the power of the single mode laser is in the relatively low range of about 150 to 200 mW, the single mode laser has not reached a satisfactory level with regards to its productivity.
The recording sensitivity of the recording material itself and laser power level are large factors in determining the recording speed during laser recording. In particular, increased laser power facilitates high speed recording of a high resolution image. In order to increase the laser power, usually, a multi-mode semiconductor laser having higher power than the single mode laser is used. Accordingly, this multi-mode semiconductor laser has a high power of 1W or more thus enabling a considerable increase in laser power of the laser head.
By using the multi-mode semiconductor laser, recording power is increased, and it becomes possible to improve the recording speed. However, there is a problem in that the multi-mode semiconductor laser has difficulties in converging a laser beam in the widthwise direction and so the laser beam cannot be converged to have a focal beam diameter as low as 20 .mu.m or less.
In the medical or printing fields, when attempts are made to record a highly accurate image having a sub-scanning pitch of about 10 .mu.m using the multi-mode semiconductor laser, adjacent beams are made to overlap with each other and overlapping portions are heated excessively, thus causing a problem in that uniform image recording is not carried out and image quality thereby deteriorates.
Typically, in a thermal transfer type (image) recording method using light-to-heat conversion action due to irradiation of a laser, a laminate, in which a thermal transfer material having an image forming layer and a thermal transfer image receiving material having an image receiving layer are laminated to each other, is irradiated with a laser. In a case where the image forming layer and the image receiving layer are in a state of being completely in contact with each other, the image forming layer and the image receiving layer each of whose temperature and ability to be set in close contact with each other have increased due to the irradiation of the laser are set in tight contact with each other. Heat is transmitted from the image forming layer to the image receiving layer, and at the same time, the surface of the image receiving layer is plasticized. Accordingly, the image receiving layer and the image forming layer can be brought in close contact with each other. By peeling the image forming layer and the image receiving layer off from each other, it is possible to transfer an image which has high sensitivity and is also uniform.
As described above, it is possible to obtain a state in which the image forming layer and the image receiving layer are set in complete contact with each other by a method in which the image forming layer and the image receiving layer are laminated to each other by passing these layers through heating rollers or pressure rollers. However, on the other hand, such a method as described above is disadvantageous in that these layers are liable to be affected by a change of temperature of the roller, or the like, processes involved become complicated, and the manufacturing cost is high. In order to solve these problems, in recent years, there has been known a method in which the image forming layer and the image receiving layer are set in contact with each other with pressure applied therebetween reduced by vacuum-suctioning (which is referred to an evacuation method hereinafter). In such a method in a case in which the smoothness of each of the surfaces of the image forming layer and the image receiving layer is too high, when the pressure applied between the image forming layer and the image receiving layer is reduced, only the peripheral portions of the surfaces whose smoothness is excessively high are set in contact with each other. Accordingly, air pockets may form at central portions of the image forming layer and the image receiving layer which are in contact with each other, thus causing poor image transfer. For this reason, in order to secure a passage for air flow when pressure is reduced and thus obtained uniform contact (adherence) between the image forming layer and the image receiving layer, the surface of the image forming layer or the image receiving layer is roughened by using a matte agent, or the like.
The vacuum-suction pressure reduction method in which the pressure applied between the image forming layer and the image receiving layer is reduced by vacuum-suctioning is preferable because, even when the image forming layer and the image receiving layer are large, these layers can be set in contact with each other uniformly. However, when the surfaces of the layers are rough, microscopic air gaps (i.e., air gaps formed at recessed portions of the roughened surface) may form at portions between the image forming layer and the image receiving layer which are set in contact with each other after this method. If these air gaps have a small size and the number of gaps is small, it is unlikely that they will cause excessive damage to an image since close contact between the image forming layer and the image receiving layer can be maintained by thermal deformation of the thermal transfer (image forming) layer during the irradiation of the laser. However, when the surfaces of the image forming layer and the image receiving layers are even rougher in order to increase the pressure reduction speed, larger microscopic air gaps form between the image forming layer and the image receiving layer. These air gaps become larger in accordance with the increase of the pressure reduction speed, thus greatly affecting the image.
If as described above, air gaps form between the image forming layer and the image receiving layer which are set in contact with each other, thermal transmission from the image forming layer to the image receiving layer is impedded. As a result, the temperature of the image receiving layer cannot increase to a temperature which suffices for plasticization of the image receiving layer. Contact between the image forming layer and the image receiving layer decreases at portions at which large microscopic air gaps are formed. Accordingly, thermal energy which was not transmitted to the image receiving layer remains at portions of the thermal transfer layer or the light-to-heat conversion layer, and the portions are heated excessively to thereby generate a gas. The air gaps expand more at the interface between the image forming layer and the image receiving layer which are in contact with each other. Due to the expansion, contact between the image forming layer and the image receiving layer further deteriorates thus causing poorer image transfer. Moreover, products of the thermal decomposition of the components of the light-to-heat conversion layer (such as binder or color material) are transferred to the image receiving layer, thus causing image defects such as a fogging.
There is a tendency for this phenomenon to be more noticeable the larger the size of materials (A2 or larger), and accordingly, image quality deteriorates greatly. Namely, it is though that in a case of large materials, in order to make these materials contact to each other uniformly, the surface roughness of the contact surface of each of the materials must be increased.
In order to achieve an increased recording speed by using a high powered laser described above in which the adjacent beams overlap, a thermal transfer material in which the layers are capable of being set in complete and uniform contact is required. This is achieved if large microscopic gaps are not formed between the thermal transfer layer, which is irradiated with lasers, and the thermal image receiving material.
As described above, the current situation is such that there has not yet been provided a thermal material which can be subjected to vacuum suctioning at high speed and simultaneously set in close contact with a thermal transfer image receiving material, and which enables formation of high quality image without impeding recording by heat even when a high output laser is used.